1. Field of the Invention
The present invention relates to high-strength, low-yield-point, cold-rolled steel sheet or strip (hereinafter referred to simply as "sheet") having excellent deep drawing property.
2. Description of the Prior Art
In recent years, demand has been rising for high-strength, cold-rolled steel sheet, particularly for automobile car bodies since such sheet is effective in reducing the car body weight and therefore contributes to fuel economy and driver safety. In the automobile industry, high-strength, cold-rolled steel sheet has been used not only for interior parts of car bodies but also for such exterior parts as hoods, trunks and fenders. Because of this, such sheet must above all have both good shape fixability after press forming, and not only a high tensile strength but also a low yield point, namely a low yield ratio (about 0.6 or less). In addition, the sheet is also required to have a high Lankford value (r) of not lower than about 1.6, a property that is required so as to preclude the pronounced appearance of surface defects, such as surface wrinkles.
Among high-strength, cold rolled steel sheets which have been developed up to now, the desired strength is obtained in some by utilizing solid solution hardening induced by carbon, manganese, phosporous, etc. and in others by utilizing the precipitation hardening induced by fine precipitates, such as TiC and NbC. Still others which have been more recently developed rely upon a dual phase structure of ferrite and martensite. However, none of the recently developed high-strength, cold rolled steel sheets can simultaneously satisfy the need for both a low yield ratio and a high r value. In all cases, one or the other of these requirements is not met. By way of example, there can be mentioned the high-strength, cold-rolled steel sheets disclosed in Japanese Patent Publication No. 31090/1975 and Japanese Publicly Disclosed Patent application No. 24952/1980. The former is a high-yield-point, high-strength, cold-rolled sheet with a high yield point that makes it inappropriate for use in applications where press forming is required. The latter is indeed a high-strength steel, but one which is highly susceptible to secondary work cracking.